Super-synchronous motor/generator

ABSTRACT

A compound motor-generator system including a first motor-generator and a second motor-generator. The first motor generator includes a stator having a set of three-phase field windings and a first rotor disposed inside and coaxial with the stator and configured to rotate relative to the stator. The second motor-generator includes a rotational stator and a second rotor coupled to a common shaft with the rotor of the first motor-generator and disposed inside and coaxial to the rotational stator. The rotational stator is configured to rotate relative to the second rotor and at a higher rotational speed than the second rotor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 16/401,430, filed May 2, 2019, the contents of which are incorporated by reference herein.

BACKGROUND

An electric motor-generator is a machine that can operate as either an electric motor or an electric generator. In the case of a compound machine, a motor-generator can perform both motor operation and generator operation at the same time. A motor-generator can be used for energy storage, or for maintaining the power quality of an electrical distribution. A motor-generator can also be used for converting voltage, frequency, and/or phase of electrical power.

SUMMARY

This specification relates to a super-synchronous motor-generator. More specifically, the disclosure relates to a motor-generator that operates at multiples of an alternating current electrical distribution system (AC system) frequency, such as two or three times the frequency.

A motor-generator that operates at super-synchronous speeds can be used to improve overall system cost and efficiency. In energy storage applications, the amount of energy storage can be vastly increased by using super-synchronous machines. Additionally, achieving super-synchronous speeds without the use of power electronics can reduce the complexity of the overall machine and the service required to properly maintain the machine.

In a super-synchronous motor-generator, two or more machines are connected, each including a rotor and a stator. The two or more machines can be located alongside one another end-to-end, or can be concentric to one another. The machines may be of different types, including synchronous, induction, reluctance, and permanent magnet.

A super-synchronous motor-generator can include two or more machines connected alongside one another, end-to-end. In this configuration, the first stator surrounds the first rotor and is stationary. The first stator flux wave rotates at synchronous speed. The first rotor is internal, and also rotates at synchronous speed, matching the frequency of the supplied AC electrical current. The first rotor is connected end-to-end (e.g., on a common or coupled shaft) to the second rotor, and they rotate at the same speed. The second stator surrounds the second rotor, and is a rotational stator. For example, the rotational second stator rotates around a common axis with the second rotor. The rotational second stator flux wave rotates at a speed that is the combination of the second rotor flux wave and the second rotor mechanical rotation speed. Thus, the rotational second stator rotates at double the synchronous speed.

A super-synchronous motor-generator can include two or more machines connected concentrically. In this configuration, the stationary stator is in the center. The first stator flux wave rotates at synchronous speed. The rotor is concentric with and outside the stationary stator, and rotates at synchronous speed along with the flux wave of the stationary stator. A movable stator is concentric with and outside the rotor. The movable stator generates a flux wave that rotates at a speed that is the combination of the rotor flux wave and the rotor mechanical rotation speed. Thus, the rotational second stator rotates at double the synchronous speed.

In general, one innovative aspect of the subject matter described in this specification can be embodied in a compound motor-generator system including a first motor-generator and a second motor-generator. The first motor generator includes a stator having a set of three-phase field windings and a first rotor disposed inside and coaxial with the stator and configured to rotate relative to the stator. The second motor-generator includes a rotational stator and a second rotor coupled to a common shaft with the rotor of the first motor-generator and disposed inside and coaxial to the rotational stator. The rotational stator is configured to rotate relative to the second rotor and at a higher rotational speed than the second rotor.

In general, another innovative aspect of the subject matter described in this specification can be embodied in a compound motor-generator system including a stator having a set of three-phase field windings; a first rotor disposed inside and coaxial with the stator, the first rotor configured to rotate relative to the stator; and a second rotor disposed inside and coaxial with the first rotor. The second rotor is configured to rotate relative to the first rotor and at a higher rotational speed than the first rotor.

The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The system may include a third motor-generator including a second rotational stator coupled to the rotational stator of the second motor-generator, and a third rotor disposed inside and coaxial with the second rotational stator and configured to rotate relative to the second rotational stator. The second rotational stator may be configured to rotate relative to the third rotor and to drive the third rotor at a higher rotational speed than the rotational speed of the second rotational stator.

In some implementations, the second rotor includes permanent magnets to supply a rotor magnetic field, and the rotational stator includes three-phase field windings configured to produce, when driven by an electrical power source, a rotational magnetic flux that rotates in a direction opposite to a direction of rotation of the second rotor.

In some implementations, the second rotor includes three-phase field windings configured to produce, when driven by an electrical power source, a magnetic flux that rotates relative to the second rotor, and the rotational stator includes permanent magnets or DC field windings configured to produce a magnetic field that is stationary relative to the rotational stator.

In some implementations, the rotational stator is coupled to a prime mover.

In some implementations, the rotational stator is coupled to a mechanical energy storage mechanism.

In some implementations, the rotor of the first motor-generator includes permanent magnets to supply a rotor magnetic field.

In some implementations, the first motor-generator and the second motor-generator are synchronous electric machines or induction machines.

The system may include an auxiliary motor coupled to the first motor-generator as a starting motor.

In some implementations, the first rotor includes permanent magnets to supply a rotor magnetic field that is stationary with respect to the first rotor, and the second rotor includes three-phase field windings configured to produce, when driven by an electrical power source, a rotational magnetic flux that rotates, relative to the second rotor, in a direction opposite to a direction of rotation of the first rotor.

In some implementations, the first rotor includes three-phase field windings configured to produce, when driven by an electrical power source, a magnetic flux that rotates relative to the first rotor, and the second rotor includes permanent magnets or DC field windings configured to produce a magnetic field that is stationary relative to the rotational stator.

In some implementations, the second rotor is coupled to a prime mover.

In some implementations, the second rotor is coupled to a mechanical energy storage mechanism.

In some implementations, the stator and the first rotor operate as a synchronous machine or as an induction machine.

In some implementations, the first rotor and the second rotor operate as a synchronous machine.

The system may include an auxiliary motor coupled to the first rotor as a starting motor. The system may include an auxiliary motor coupled to the second rotor as a starting motor.

The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams of exemplary implementations of super-synchronous motor-generators in an end-to-end configuration.

FIG. 2 is a diagram of an exemplary super-synchronous motor-generator in a concentric configuration.

FIG. 3 is a diagram of an exemplary super-synchronous motor-generator connected to a flywheel.

FIG. 4 is a diagram of an exemplary super-synchronous motor-generator including three connected machines.

FIGS. 5A to 5D are cross-sectional diagrams of the embodiments of super-synchronous motor-generators shown in FIGS. 1 to 4 .

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

In a grid-connected power generation, or energy storage, design, there is generally a combination of electronic and electromechanical devices deployed between the grid and the prime mover or power source/storage. These devices are used to condition and transfer the power and energy in a controlled fashion. For rotational systems, this chain of electrical and electronic stages typically consists of an isolation transformer, a motor-generator, power electronics, and a gearbox to match the prime mover speed and the motor-generator speed. Alternatively, power electronics can be used to change the frequency of a motor-generator to match the frequency of the grid, typically using back to back AC/DC power converters.

A motor-generator that operates at super-synchronous speeds can be used to improve overall system cost and efficiency. Additionally, achieving super-synchronous speeds without the use of power electronics may reduce the complexity of the machine and the service required to properly maintain the machine.

This specification relates to a super-synchronous motor-generator. More specifically, the disclosure relates to a motor-generator that can operate at multiples of an AC system frequency, such as two or three times the frequency.

In a super-synchronous motor-generator, two or more machines are connected, each including a rotor and a stator. In some examples, the rotor and stator windings are fed from a three-phase AC electrical grid. In some examples, one or more of the rotors include a permanent magnet. The two or more machines can be located alongside one another end-to-end, or can be concentric to one another.

FIGS. 1A and 1B are diagrams of an exemplary super-synchronous motor-generator 100 in an end-to-end configuration. The motor-generator 100 includes a first machine 100 a coupled to a second machine 100 b. The first machine 100 a includes a stationary housing 110 a that encases a first stator 120 a and a first rotor 130 a. The second machine 100 b includes a housing 110 b that encases a second stator 120 b and a second rotor 103 a. The second stator 120 b of the second machine 100 b is configured as a rotational stator. For example, the second stator 120 b can be mounted on bearings 140 b instead of being fixed to the housing 110 b. As such, the second stator 120 b is free to rotate relative to the housing 110 b and/or relative to, and independent of, the second rotor 130 b.

Although illustrated as two separate housings 110 a, 110 b, both machines 100 a, 110 b can be enclosed in a common housing. The motor generator 100 can be considered as a compound machine because, as noted above, the first machine 100 a is coupled to the second machine 100 b. For example, first machine 100 a can be coupled to second machine 100 b by coupling the first rotor 130 a to the second rotor 130 b, e.g., by a common shaft or by a mechanical coupling between the two rotors.

In the example in FIGS. 1A and 1B, both first machine 100 a and second machine 100 b can be configured as synchronous machines. In a synchronous machine, the mechanical rotation of a rotating component (e.g., a rotor) aligns with a rotating magnetic field, i.e., the mechanical rotational speed is substantially equal to the rotational speed of a rotating magnetic field produced by a stator.

The motor-generator 100 can operate as a motor. Briefly, when operating as a motor, electrical power is supplied to stator 120 a of the first machine 100 a causing rotor 130 a to mechanically rotate. The rotation of rotor 130 a causes rotor 130 b of the second machine 100 b to rotate, which (as described in more detail below) cause the rotational stator 120 b to rotate at twice the mechanical speed of rotor 130 b.

In more detail, when the motor-generator 100 operates as a motor, electrical power is supplied to first machine 100 a. The source of the electrical power can be, for example, a three-phase AC electrical distribution system. The electrical power source supplies electrical current through power cables 115 a to the armature windings of first stator 120 a (e.g., as illustrated by element 502 in FIG. 5A). The electrical current supplied to the armature windings of first stator 120 a creates a magnetic flux that rotates at an angular speed wo.

The first stator 120 a magnetic flux rotates at synchronous angular speed ω_(ϕa), which means that the rotational speed of the magnetic flux matches the frequency of the power supplied from the AC system. For example, if the frequency of the AC system is 60 Hertz (Hz), the first stator 120 a magnetic flux rotates at a speed ω_(ϕa) of 3600 revolutions per minute (rpm), or a factor of 3600 rpm, where 3600 rpm is divided by the number of pole pairs. Though the first stator 120 a magnetic flux rotates at synchronous speed, as noted above, the first stator 120 a is mechanically stationary.

A first rotor 130 a rotates to align with the first stator 120 a magnetic flux at a mechanical rotational speed equal to the rotational speed of the stator magnetic flux. The first rotor 130 a can be constructed with either field windings (e.g., as illustrated by element 512 in FIG. 5C) or one or more permanent magnets (e.g., as illustrated by element 504 in FIG. 5A) to produce a stationary magnetic field on the rotor field that follows or “locks onto” (e.g., in a synchronous motor design) the rotating magnetic flux 121 of the first stator 120 a. The rotating magnetic flux produced by the first stator 120 a causes the first rotor 130 a to rotate at an angular speed ω_(ra)=ω_(ϕa).

The first rotor 130 a is mechanically connected with the second rotor 130 b. For example, the first rotor 130 a and second rotor 130 b can be on a common shaft, or can be mechanically coupled. The mechanical rotation of the first rotor 130 a drives the second rotor 130 b at a speed ω_(rb) that is equal to ω_(ra).

In the example illustrated in FIG. 1A, the second rotor 130 b includes a set of three-phase windings (e.g., as illustrated by element 506 in FIG. 5B) that achieves a rotating magnetic flux. The three-phase windings can be driven to create a rotational magnetic flux 131 that rotates at ω_(ϕb) with respect to the second rotor 130 b. For example, the magnetic flux 131 of the second rotor 130 b can be driven through power cables 115 b coupled to a power source. The field windings of the second rotor 130 b can be driven by the same or a different power source as that used to drive the field windings of the first stator 120 a. The three-phase windings are driven to create a rotational magnetic flux 131 that rotates in the same direction ω_(ϕb) as the rotor itself rotates, such that the second rotor's magnetic flux 131 rotates at, for example, twice the synchronous speed of the first machine 110 a, e.g., ω_(ϕb)=2ω_(ϕb).

The rotational second stator 120 b is mounted on high-speed bearings 140 b that allow the rotational second stator 120 b to mechanically rotate with respect to the stationary housing 110 b. The second stator 120 b includes either field windings (e.g., as illustrated by element 518 in FIG. 5D) driven by a DC source or a permanent magnet (e.g., as illustrated by element 508 in FIG. 5B) to generate a stationary magnetic field (e.g., stationary with respect to the second stator 120 b). The stator field then “locks onto” (e.g., in a synchronous motor design) the second rotor's rotating magnetic flux 131 causing the rotational stator 120 b to rotate at the same speed as the second rotor's rotating magnetic flux 131, e.g., the speed (ω_(sb)) of the rotational stator is twice the speed (ω_(rb)) of the second rotor 130 b (ω_(sb)=2ω_(rb)). Further, assuming that both machines 100 a and 100 b are operated as synchronous machines, the rotational second stator 120 b will rotate at twice the synchronous speed of the first machine 110 a.

For example, if the second rotor 130 b rotates at 3600 rpm in a clockwise direction, the second rotor 130 b magnetic flux 131 also rotates in a clockwise direction but at 7200 rpm. The rotational second stator 120 b stationary field will follow the rotating magnetic flux 131 of the second rotor 130 b causing the rotational second stator 120 b to also rotate at 7200 rpm in the clockwise direction.

In another example illustrated in FIG. 1B, the second rotor 130 b can be configured with a stationary magnetic field rather than a rotating magnetic field. For example, the second rotor 130 b can include field windings (e.g., as illustrated by element 512 in FIG. 5C) driven by a DC source or a permanent magnet (e.g., as illustrated by element 504 in FIG. 5A) to produce a magnetic field that is stationary with respect to the second rotor 130 b. However, the second rotor's magnetic field will still rotate at a speed of ω_(rb) with respect to housing 110 b as the second rotor 130 b is driven by the first rotor 130 a. In such examples, super-synchronous speed of the rotational second stator 120 b can be achieved with respect to the first machine 100 a by driving field windings (e.g., as illustrated by element 514 in FIG. 5C) on the second stator 120 b to produce a rotational magnetic flux 132 that rotates at a speed ω_(ϕb) with respect to the rotational second stator 120 b. Specifically, the field windings of the second stator 120 b are driven so as to produce a magnetic flux 132 that rotates at the same speed as the magnetic flux 121 of the first stator 120 a, but in an opposite direction; e.g., ω_(ϕb)=−ω_(ϕa). Hence, by extension, the second stator's magnetic flux 132 is also driven in a direction opposite to the rotation of the second rotor 130 b. Thus, when the magnetic flux 132 “locks onto” the stationary field of the second rotor 130 b, as the second rotor rotates, the interaction between the second stator's magnetic flux 132 and the magnetic field of the second rotor 130 b will tend to “push” the rotational second stator 120 b at twice the speed at which the second rotor 130 b rotates. In other words, the speed (ω_(sb)) of the rotational stator will be twice the speed (ω_(rb)) of the second rotor 130 b (ω_(sb)=2ω_(rb)).

For example, if the second rotor 130 b rotates at 3600 rpm in a clockwise direction, the second rotor's 130 b magnetic field will also rotates in a clockwise direction at 3600 rpm. The rotational second stator 120 b is driven to produce a magnetic flux that rotates relative to the second stator 120 b at 3600 rpm, but in the counter-clockwise direction. For example, the phases of the field windings of the second stator 120 b can be coupled to power cables 115 b according to a phase sequence that produces an opposite direction of field rotation as compared to the phase sequence connection between the power cables 115 a and the field windings of the first stator 120 a. Thus, if the second rotor were held stationary, and the rotational second stator 120 b allowed to freely rotate, the second stator's magnetic flux 132 would tend to push the second stator 120 b in the clockwise direction at 3600 rpm. However, when this effect is combined with the rotation of the second rotor 130 b as driven by the first rotor 130 a, the second rotor 120 b drives the rotational second stator 120 b at 7200 rpm in the clockwise direction (e.g., ω_(sb)=ω_(rb)−ω_(ϕb)=2ω_(rb), where ω_(rb)=ω_(ϕa) and ω_(ϕb)=−ω_(ϕa)).

In some applications, the rotational second stator 120 b can be coupled to a mechanical energy storage device, such as a flywheel. Additional details about energy storage implementations are described below in reference to FIG. 3 .

When the motor-generator 100 operates as a generator, a prime mover is connected to, or coupled with, the rotational second stator 120 b. The prime mover can be, for example, any type of turbine such as a steam turbine or gas turbine, or any type of engine such as a diesel engine or gasoline piston engine. Because the super-synchronous motor generator 100 can operate at super-synchronous speeds, the motor generator 100 can produce a 60 Hz electrical output while the prime mover is operated at speeds in excess of 3600 rpm without the need for reduction gears or frequency converters. For example, a prime mover can be driven at 7200 rpm, which is two times the fundamental operating frequency of a 3600 rpm, or 60 Hz, system. Synchronous devices rotate at f/N, where f is the electrical frequency and N is the number of pole pairs in the device, nominally 1 for high speed motor-generators. The prime mover can be driven at 7200 rpm without the need for reduction gears.

When operating as a generator, the super-synchronous motor generator 100 operates in a manner similar to that described above in reference to FIGS. 1A and 1B except that the prime mover provides the motive force to the rotational second stator 120 b, and the relative motion of the first rotor 130 a with respect to stator coils of the first stator 120 a induces an output voltage/current on the stator coils. The electric current then feeds an electrical load through power cables 115 a. The load may be, for example, a three-phase AC electrical distribution system.

In some examples, a supplemental electrical power source can be used to power the field windings of the second stator 120 b (or the second rotor 130 b, depending on the configuration of the second machine 100 b) e.g., when the motor generator 100 is started up.

FIG. 2 includes both an exploded-view diagram and a fully assembled-view diagram of an exemplary super-synchronous motor-generator 200 in a concentric configuration. Motor-generator 200 is similar to motor-generator 100, in that it is a compound machine, however, instead of the machines being coupled end-to-end as in motor-generator 100, the machines are constructed concentrically, and share a common rotor. For example, motor-generator 200 includes an inner stator 220 a, surrounded by a rotor 230 coaxially mounted to the inner stator on a set of bearings 250, and an outer stator 220 b coaxially mounted to the rotor 230 on a set of bearings 250. Either the inner stator 220 a or the outer stator 220 b can be configured as a rotational stator. That is, either the inner stator 220 a or the outer stator 220 b can be mounted on bearings (e.g., bearings 240 and 250) such that it is free to mechanically rotate relative to the housing 210 and relative to the rotor 230. While described as a second stator for consistency with respect to the implementations shown and described in reference to FIGS. 1A and 1B, the rotational stator (e.g., either second stator 220 b or first stator 220 a, depending on the configuration) can be considered as a second rotor. For example, rotational stator 220 b can be considered as a second rotor disposed within a hollow first rotor 230 and supported on compound bearings 250.

The first stator 220 a includes three-phase field windings (e.g., armature windings as illustrated by element 502 in FIG. 5A)) configured to generate a rotational magnetic flux (ω_(ϕa)). For example, the first stator windings can be driven by an electrical power source through power cables 215 a (when operating as a motor). When operating as a generator, an output voltage/current is induced on the first stator coils through electro-magnetic interaction with a magnetic field produced by the rotor 230.

As discussed with respect to the implementations depicted in FIGS. 1A and 1B, in some implementations, the rotor 230 can include either field windings (e.g., as illustrated by element 512 in FIG. 5C) driven by a DC source or a permanent magnet (e.g., as illustrated by element 504 in FIG. 5A) to generate a stationary magnetic field (e.g., stationary with respect to the rotor 230), while the second stator 220 b includes three-phase windings (e.g., armature windings as illustrated by element 516 in FIG. 5D) to generate a rotating magnetic flux. The second stator windings can be driven by an electrical power source through power cables 215 b.

In some implementations, the rotor 230 can include three-phase field windings to generate a rotating magnetic flux, (e.g., as illustrated by element 502 in FIG. 5A) while the second stator 220 b includes either field windings (e.g., as illustrated by element 512 in FIG. 5C) driven by a DC source or a permanent magnet (e.g., as illustrated by element 504 in FIG. 5A) to generate a stationary magnetic field (e.g., stationary with respect to the second stator 220 b). The rotor windings can be driven by an electrical power source through power cables 215 b.

Similar to motor-generator 100 described above, the exemplary motor-generator 200 in FIG. 2 can operate as a motor or a generator. When operating as a motor, electrical power is supplied to first machine 200 a, causing machine 200 b to mechanically rotate. When operating as a generator, a prime mover couples to machine 200 b, causing machine 200 a to produce electrical power.

The operations of motor generator 200 are similar to those of the two implementations of motor generator 100 described above. Thus for brevity, only the motor operations of the permanent magnet rotor 230 implementation of motor generator 200 are described below. When operating as a motor, electrical power is supplied to the first stator 220 a. The source of the electrical power can be, for example, a three-phase AC electrical distribution system. The electrical power source supplies electrical current through power cables 215 a to the armature windings of the first stator 220 a. The electrical current supplied to the armature windings of first stator 220 a creates a magnetic flux that rotates at an angular speed ω_(ϕa).

The first stator 220 a magnetic flux rotates at synchronous angular speed ω_(ϕa), which means that the rotational speed of the magnetic flux matches the frequency of the power supplied from the AC system. For example, if the frequency of the AC system is 60 Hertz (Hz), the first stator 220 a magnetic flux rotates at a speed ω_(ϕa) of 3600 revolutions per minute (rpm). Though the first stator 220 a magnetic flux rotates at synchronous speed, as noted above, the first stator 220 a itself is mechanically stationary.

A rotor 230 rotates to align with the first stator 220 a magnetic flux at a mechanical rotational speed equal to the rotational speed of the first stator magnetic flux. The rotor 230 can be constructed with either field windings or one or more permanent magnets to produce a stationary magnetic field on the rotor. The field follows or “locks onto” (e.g., in a synchronous motor design) the rotating magnetic flux 221 of the first stator 220 a. The rotating magnetic flux produced by the first stator 220 a causes the rotor 230 to rotate at an angular speed ω_(r)=ω_(ϕa).

The rotor 230 also rotates at the angular speed ω_(r) with respect to the second stator 220 b. Thus, the rotor's magnetic field also rotates at a speed of ω_(r) with respect to the second stator 220 b. In such examples, super-synchronous speed of the rotational second stator 220 b can be achieved with respect to the rotor 230 by driving field windings on the second stator 220 b to produce a rotational magnetic flux 232 that rotates at a speed ω_(ϕb) with respect to the rotational second stator 220 b. Specifically, the field windings of the second stator 220 b are driven so as to produce a magnetic flux 232 that rotates at the same speed as the magnetic flux 221 of the first stator 220 a, but in the opposite direction; e.g., ω_(ϕb)=−ω_(ϕa). Hence, by extension, the second stator's magnetic flux 232 is also driven in a direction opposite to the rotation of the rotor 230. Thus, when the magnetic flux 232 “locks onto” the magnetic field of the rotor 230, as the second rotor rotates, the interaction between the second stator's magnetic flux 232 and the magnetic field of the of the rotor 230 will tend to “push” the rotational second stator 220 b at twice the speed at which the rotor 230 rotates. In other words, the speed (ω_(sb)) of the rotational stator will be twice the speed (ω_(r)) of the rotor 230 (ω_(sb)=2ω_(r)).

For example, if the rotor 230 rotates at 3600 rpm in a clockwise direction, the rotor's 230 magnetic field will also rotates in a clockwise direction at 3600 rpm. The rotational second stator 220 b is driven to produce a magnetic flux that rotates relative to the second stator 220 b at 3600 rpm, but in the counter-clockwise direction. For example, the phases of the field windings in the second stator 220 b can be coupled to power cables 115 b according to a phase sequence that produces an opposite direction of field rotation as compared to the phase sequence connection between the power cables 115 a and the field windings of the first stator 120 a. Thus, if the rotor 230 were held stationary, and the rotational second stator 220 b allowed to freely rotate, the second stator's magnetic flux 232 would tend to push the second stator 220 b in the clockwise direction at 3600 rpm. However, when this effect is combined with the rotation of the rotor 230 as driven by the first stator 220 a, the rotor 230 drives the rotational second stator 220 b at 7200 rpm in the clockwise direction (e.g., ω_(sb)=ω_(r)−ω_(ϕb)=2ω_(r), where ω_(r)=ω_(ϕa) and ω_(ϕb)=−ω_(ϕa)).

Although the examples in FIGS. 1A-2 are described in reference to synchronous machines, a super-synchronous motor-generator can be constructed as induction or reluctance machines in other implementations. The rotational speeds can vary depending on the types of machines used. For example, in a motor-generator that includes induction machines, the motor-generator can achieve intermediate speeds that are in between multiples of synchronous speed, or below synchronous speed. Synchronous machines are generally not self-starting machines. If a rotor is stationary, it cannot instantly follow the rotation of the stator magnetic flux due to inertia. Therefore, a super-synchronous motor-generator that includes synchronous motors and/or synchronous generators can include a supplemental starting mechanism.

For example, one approach to starting a synchronous machine is to energize the stator from the AC electrical distribution system, and meanwhile to short the rotor windings so that the machine operates as an induction machine for starting. The machine operates as an induction machine until it comes up to near synchronous speed. When the slip is small, and the phase angle is near zero, the rotor field winding is excited, and the rotor rotation synchronizes with the stator flux rotation.

Another method for starting a synchronous machine is to use a small motor, e.g., an auxiliary or “pony” motor (e.g., as illustrated by element 520 in FIG. 5D). The pony motor can be used to accelerate a rotor to near synchronous speed before the stator winding connects to the AC electrical distribution system. When the stator windings switch on, the machine shifts to synchronous operation and the pony motor decouples. For example, auxiliary starting motors can be coupled to any of the rotors or rotational stators of the motor-generators described above to bring the respective rotational component up to its operating speed. For instance, referring to FIG. 1B, an auxiliary motors can be coupled to rotors 130 a/130 b (e.g., one auxiliary motor can be used to start both rotors 130 a/130 b since they are coupled to a common shaft) to bring rotors 130 a/130 b up to approximately 3600 rpm when starting the super-synchronous motor generator 100. An auxiliary motor can be coupled to the rotational stator 120 b of the second machine 100 b to bring the rotational stator 120 b up to 7200 rpm when starting the super-synchronous motor generator 100.

FIG. 3 is an illustration of an exemplary super-synchronous motor-generator connected to a flywheel (motor-generator-flywheel). A flywheel is a spinning disk used to store energy as mechanical kinetic energy. FIG. 3 shows a super-synchronous motor-generator-flywheel 300 in an end-to-end configuration.

In the example of FIG. 3 , when motor-generator-flywheel 300 operates as a motor in energy-storage mode, electrical power is supplied from a 60 Hz AC electrical distribution system to the first stator 320 a of first machine 300 a through electrical power cables 310 a. The first rotor 330 a of first machine 300 a rotates to match the AC frequency at a speed of 3600 rpm. The second rotor 330 b is connected or coupled to the first rotor 330 a. The second rotor 330 b also rotates at 3600 rpm, causing the rotational second stator 320 b to rotate at 7200 rpm. The rotational second stator 320 b is connected or coupled to a flywheel 350, which also rotates at 7200 rpm. The flywheel 350 is a disk that stores kinetic energy by spinning.

The amount of energy stored in a flywheel is proportional to the square of the speed of rotation. Therefore, doubling the speed of rotation of a flywheel multiplies the amount of energy storage by four. This means that the flywheel 350 spinning at 7200 rpm can store four times as much energy as the same flywheel 350 spinning at 3600 rpm. When electric power is needed, such as when there is a power outage in the AC system, the super-synchronous motor-generator-flywheel 300 can convert the energy stored in the flywheel 350 back to electrical energy.

In the example of FIG. 3 , when motor-generator-flywheel 300 operates as a generator in electrical generation mode, electrical power is not provided through the power cables 310 a, possibly because of a power outage. The flywheel 350 continues to rotate at 7200 rpm due to its angular momentum. Flywheels can be designed to minimize friction and air resistance. This can enable flywheels to rotate for long periods of time, such as hours or days, without being driven by a motor.

The flywheel 350 acts as a prime mover, and rotates the rotational second stator 320 b at 7200 rpm. The magnetic flux created by the rotational second stator 320 b causes the second rotor 330 b to rotate at half the speed, or 3600 rpm.

The second rotor 330 b is coupled to the first rotor 330 a, which also rotates at 3600 rpm. The field windings on the first rotor 330 a induce a rotating magnetic field in the armature windings on the first stator 320 a. This produces a 60 Hz AC electric current that is then provided to the AC system through the power cables 310 a. In this way, the super-synchronous motor-generator-flywheel 300 can provide backup power to an AC system during a power outage.

A super-synchronous motor-generator-flywheel can store more energy than a slower, synchronous motor-generator-flywheel. A two-machine motor-generator-flywheel that operates at double the frequency of an AC system may store approximately four times as much energy compared to a synchronous machine. A three-machine motor-generator-flywheel operating at triple the frequency of an AC system may store approximately nine times as much energy compared to a synchronous machine.

Super-synchronous speeds can also enable energy storage with smaller and lighter flywheels. The amount of energy storage in a flywheel is proportional to the moment of inertia, or effective mass, of a flywheel, and to the square of the rotational speed of the flywheel. Therefore, a flywheel that doubles its rotational speed can have one-fourth the effective mass while storing the same amount of energy.

Super-synchronous motor-generator-flywheels can have many applications. For example, flywheels can store energy from power sources that have peak and off-peak periods, such as solar panels and wind turbines. When there is more electricity supply than demand, AC systems can feed the excess energy to flywheels. For example, flywheels can store energy from solar panels when the sun is shining, and from wind turbines when winds are strong. Flywheels can then release the energy when there is a need, such as at night or when the winds are calm.

Another application of super-synchronous motor-generator-flywheels is maintaining electrical power quality. Power quality is the measure of voltage and frequency disturbances in an electrical system. A motor-generator-flywheel can be connected to an electrical system near an electrical power source such as a power plant. If there are small disruptions to the electrical system, such as voltage sag or momentary interruptions, the motor-generator-flywheel can provide short-term power continuity to maintain stability in the electrical system.

The example motor-generator-flywheel 300 can also be built in a concentric configuration, as shown in FIG. 2 . In this configuration, the flywheel 350 connects to the outermost rotating component. In the example of FIG. 2 , the flywheel would connect to the rotating stator 220 b.

Though the example in FIG. 3 shows the rotational second stator 320 b connected to a flywheel 350, the flywheel 350 can be replaced with a prime mover such as a turbine or engine rotating at 7200 rpm. The operation of the motor-generator 300 with a prime mover attached to the rotational second stator 320 b is similar to the operation of the motor-generator-flywheel 300 operating in power generation mode.

FIGS. 1A and 2A illustrate super-synchronous motor-generators that include two connected machines. Additional machines can be connected end-to-end or concentrically to achieve additional multiples of synchronous frequency, such as three or four times synchronous frequency.

FIG. 4 is an illustration of an exemplary super-synchronous motor-generator including three connected machines. For example, super-synchronous motor-generator 400 includes three machines, 400 a, 400 b, and 400 c, in an end-to-end configuration. The rotor 430 b of the second machine 400 b is connected or coupled to the rotor 430 a of the first machine. The rotational stator 420 b of the second machine 400 b is connected or coupled to the rotational stator 420 c of the third machine 400 c.

When motor-generator 400 operates as a motor, electrical power is supplied from a 60 Hz AC electrical distribution system to first machine 400 a through electrical power cables 410 a. The rotor 430 a of the first machine 400 a rotates to match the AC frequency at a speed of 3600 rpm. The rotor 430 b of the second machine 400 b also rotates at 3600 rpm, causing the rotational stator 420 b of the second machine 400 b to rotate at 7200 rpm.

The rotational stator 420 b of the second machine 400 b, rotating at 7200 rpm, causes the rotational stator 420 c of the third machine 400 c to rotate at 7200 rpm.

The rotational speed of the rotor 430 c of the third machine 400 c is the sum of the speed of the rotational stator 420 c of the third machine 400 c (e.g., 7200 rpm) and the rotational stator's magnetic flux angular speed (e.g., 3600 rpm—if driven from a 60 Hz source). Therefore, the rotor 430 c of the third machine 400 c rotates at 10,800 rpm.

In an energy-storage application, a motor-generator-flywheel rotating at 10,800 rpm may store approximately nine times as much energy as a motor-generator-flywheel rotating at 3600 rpm. Thus, adding additional machines to a super-synchronous motor-generator results in higher rotational speeds and much higher storage capacity.

In the example of FIG. 4 , when motor-generator 400 operates as a generator, a prime mover drives the third rotor 430 c at super-synchronous speeds. The prime mover can be, for example, any type of flywheel, engine, or turbine. In the example of FIG. 4 , the prime mover drives the third rotor 430 c at 10,800 rpm, however, the electrical output at the first machine 400 a will be 60 Hz (e.g., equivalent to a 3600 rpm synchronous speed).

The example super-synchronous motor-generator 400 can include more than three machines, in either the end-to-end or concentric configurations, in order to achieve higher rotational speeds.

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

Thus, particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous. 

The invention claimed is:
 1. An electrical machine comprising: a housing; a first stator having a first set of field windings connected to an electric grid; a first rotor disposed inside and coaxial with the first stator, wherein, in a first operating mode, the first stator is configured to electrically drive rotation of the first rotor relative to the first stator when the first set of field windings is energized; a second stator mounted on bearings configured to permit rotation of the second stator relative to the housing; and a second rotor mechanically coupled to the first rotor, wherein, in the first operating mode, the first rotor is configured to mechanically drive rotation of the second rotor during rotation of the first rotor, wherein: the second rotor is disposed inside and coaxial to the second stator; and the second rotor is configured to electrically drive rotation of the second stator when a second set of field windings are energized during rotation of the second rotor, the second set of field windings being connected to the electric grid and integrated with the second rotor.
 2. The electrical machine of claim 1, wherein, in a second operating mode, the first rotor is configured to induce electrical current in the first set of field windings during rotation of the first rotor relative to the first stator and the second rotor is configured to mechanically drive rotation of the first rotor during rotation of the second rotor.
 3. The electrical machine of claim 1, wherein, in the first operating mode, the second rotor is configured to electrically drive rotation of the second stator at a faster rotational speed than the rotational speed of the second rotor when the second set of field windings are energized during rotation of the second rotor.
 4. The electrical machine of claim 1, wherein, in the first operating mode, the second rotor is configured to electrically drive rotation of the second stator at approximately twice the rotational speed of the second rotor when the second set of field windings are energized during rotation of the second rotor.
 5. The electrical machine of claim 1, wherein the first rotor is connected end-to-end with the second rotor.
 6. The electrical machine of claim 1, wherein the second stator is coupled to a mechanical energy storage device configured to store energy from the electric grid as mechanical energy.
 7. The electrical machine of claim 1, wherein: the first stator and the first rotor comprise a first synchronous machine; and the second stator and the second rotor comprise a second synchronous machine.
 8. An electrical machine comprising: a housing; a first stator having a first set of field windings connected to an electric grid; a first rotor disposed inside and coaxial with the first stator, wherein, in a first operating mode, the first rotor is configured to induce electrical current in the first set of field windings during rotation of the first rotor relative to the first stator to feed electrical current to the electric grid through the first set of field windings; a second rotor mechanically coupled to the first rotor, wherein, in the first operating mode, the second rotor is configured to mechanically drive rotation of the first rotor during rotation of the second rotor; and a second stator mounted on bearings configured to permit rotation of the second stator relative to the housing, the second stator being configured to be mechanically rotated by a prime mover in the first operating mode, wherein: the second rotor is disposed inside and coaxial to the second stator; and the second stator is configured to electrically drive rotation of the second rotor when a second set of field windings are energized during rotation of the second stator, the second set of field windings being integrated with the second rotor or the second stator.
 9. The electrical machine of claim 8, wherein the second set of field windings is integrated with the second rotor.
 10. The electrical machine of claim 8, wherein the second set of field windings is integrated with the second stator.
 11. The electrical machine of claim 8, wherein, in a second operating mode, the first stator is configured to electrically drive rotation of the first rotor relative to the first stator when the first set of field windings is energized by the electric grid and the first rotor is configured to mechanically drive rotation of the second rotor during rotation of the first rotor.
 12. The electrical machine of claim 8, wherein, in the first operating mode, the second stator is configured to electrically drive rotation of the second rotor at a slower rotational speed than the rotational speed of the second stator when the second set of field windings are energized.
 13. The electrical machine of claim 8, wherein, in the first operating mode, the second stator is configured to electrically drive rotation of the second rotor at approximately half the rotational speed of the second stator when the second set of field windings are energized during rotation of the second stator.
 14. The electrical machine of claim 8, wherein the first rotor is connected end-to-end with the second rotor.
 15. The electrical machine of claim 8, wherein the prime mover comprises a turbine.
 16. The electrical machine of claim 8, wherein: the first stator and the first rotor comprise a first synchronous machine; and the second stator and the second rotor comprise a second synchronous machine.
 17. The electrical machine of claim 8, wherein the prime mover comprises a mechanical energy storage device configured to store energy from the electric grid as mechanical energy.
 18. A method of operating an electrical machine comprising a first stator, a first rotor disposed inside and coaxial with the first stator, a second stator mounted on bearings configured to permit rotation of the second stator relative to a housing, and a second rotor disposed inside and coaxial with the second stator, the method comprising: energizing a second set of field windings integrated with the second stator or the second rotor; mechanically rotating the second stator by a prime mover to electrically drive rotation of the second rotor, wherein the second rotor is mechanically coupled to the first rotor and mechanically drives rotation of the first rotor during rotation of the second rotor; and feeding an electric grid with electrical current induced in a first set of field windings integrated with the first stator, the electrical current being induced by rotation of the first rotor relative to the first stator. 